If the server you were using to download was capable of consistent high speed, PB would show over 100% of your provisioned speed for the first 20 or so MB and then level out delivering the full speed of your service.

Over how long a period of time is this graph? Do you see the same patter with all downloads, different servers, wired vs wireless, various PCs etc?

I did another test. This time I used just command line ftp. The file (1G) is located at ftp.windstream.net (my old ISP) The two attached graphs show last 60s. One is from ubuntu (12.04), the other from Windows 8. Both show the same pattern.

I swear i've seen a download pattern like this before, can't remember where but it had something to do with traffic shaping.

are you *sure* your not doing any local traffic shaping, you've tried various download sites, from different computers on your LAN (at different times of the day) and while directly connected to your cable modem?

you've also ran shaperprobe tests to verify some of the basic stats of your connection, too, right?

this might also be a long shot, but any neighbors you're friendly with have comcast (residential or business) as well? maybe you can try to reproduce this download pattern with one of them, too.

It's possible that you have a congested node... IIRC way back they added in a feature that would throttle your connection to 50% if the node was overloaded and you were in the top percentage of users. Your download would be the perfect example of this because it's a long sustained connection eating up all of the bandwidth. By throttling it slightly everyone else's traffic can remain unaffected while just making your download take a bit longer.

quote:In line with their traffic management website Comcast has confirmed to us they've installed their new broadband throttling system across all markets. The system, which we first profiled back in September, throttles a user's connection if a particular CMTS port is congested, and if that user has been identified as a primary reason why. This two-condition throttling system replaces Comcast's old, FCC-criticized system of using forged TCP packets to throttle upstream P2P services for all users, regardless of consumption.

According to Comcast's filings (pdf) with the FCC, they've deployed new hardware and software close to the company's Regional Network Routers (RNRs). This hardware will flip a user from the standard "Priority Best-Effort" traffic (PBE) to lower quality of service (QoS) "Best-Effort" traffic (BE) for fifteen minutes if they're a major reason congestion exists.

While certainly a slightly more transparent system to those paying attention, the new system is probably going to confuse the American public, many of whom don't even know what a gigabyte is. Comcast used a bus metaphor to explain the difference between best effort and priority best effort traffic to the FCC:If there is no congestion, packets from a user in a BE state should have little trouble getting on the bus when they arrive at the bus stop. If, on the other hand, there is congestion in a particular instance, the bus may become filled by packets in a PBE state before any BE packets can get on. In that situation, the BE packets would have to wait for the next bus that is not filled by PBE packets.Comcast says that sustained use of 70% of your up or downstream throughput triggers the BE state, at which point you'll find your traffic priority lowered until your usage drops to 50% of your provisioned upstream or downstream bandwidth for "a period of approximately 15 minutes." A throttled Comcast user being placed in a BE state "may or may not result in the user's traffic being delayed or, in extreme cases, dropped before PBE traffic is dropped."

Note that upstream and downstream bandwidth are managed separately. Also note that the differentiation between PBE and BE traffic occurs in two millisecond increments. According to Comcast, even if the packets for a best effort throttled user missed 50 "busses," the delay would only be about one-tenth of a second.

In addition to the new throttling system, Comcast has also a 250GB monthly usage cap for all users. As we mentioned last Friday, Comcast has confirmed that a web portal-based bandwidth tracker is currently in beta among Comcast employees -- but has yet to give an official launch date. A Comcast insider had previously given us leaked screenshots of the monitor, and said it was originally scheduled to go live on January 5 (today).

Comcast has confirmed to us that they've completed the upgrade to the new system.

I saw this a LONG time ago on a DSL line. It was due to a mismatched buffer size between the DSLAM and the ISP's equipment or something like that.

It definitely looks to me like some sort of misconfigured traffic shaping.

Would it be possible for you to get a pcap dump (tcpdump/wireshark) and post it here? It should be sufficient to capture for only 1 or 2 cycles of the sawtooth.

I did notice with some TCP stacks or congestion algorithms, powerboost can cause issues where the speed constantly ramps up and drops off. But you're observing this on multiple OSes, so that's likely not the issue.--"Women. Can't live with 'em, pass the beer nuts." -Norm

Well, so much for trying to capture packets tonight. Ran a test download from same site as yesterday and got almost perfect flat line ;) Also ran shaperprobe:lac@heron:~/Desktop/shaperprobe$ ./prober -vDiffProbe release. January 2012.Shaper Detection Module.

Let me rephrase that... Powerboost will reappear on business accounts after the initial "boost" whereas on residential accounts it's only available for the first part of a download.

That's what I remember reading somewhere anyway.

The only difference I've noticed with PowerBoost on business is that it's basically uncapped. For example, residential 25/4 Blast hits about 35/6 with PB. I have business 22/5 and just now it hit 93/20. Other than that, it's the same. It basically looks like the picture mrschultz02 posted above, just with a much higher initial spike.--KI6RIT

Next time it happens try doing three or more downloads at the same time, to three different servers - if the combined download speed sawtooths, it's something specific to your end, or the path leading up to your end inside of comcast. If not, it's on the other end.--My place : »www.schettino.us